1. Field of the Invention
The present invention generally relates to a tuner suitably for receiving a television signal, especially a cable television (hereafter referred to as a CATV) signal.
2. Description of the Related Art
For a CATV receiving tuner, a tuner of double converting is known. The double converting tuner has two mixing circuits and two local oscillating circuits for converting a frequency twice, thereby presenting drawbacks of complicating the mechanical construction and increasing the fabrication cost. Therefore, a single converting tuner has recently come to be used as the CATV receiving tuner.
FIG. 4 is a block diagram illustrating a conventional single converting tuner used for CATV reception. The single converting tuner shown in FIG. 4 has a generally the same constitution as that of a tuner for receiving a general television broadcasting transmitted not by cable but by electromagnetic wave (the television broadcasting of this type is hereafter referred to as ground wave television).
To be more specific, as shown in FIG. 4, in the conventional tuner, an input tuning circuit 31, a high frequency amplifying circuit 32, an interstage tuning circuit 33, and a mixing circuit 34 are sequentially interconnected in a cascade manner. A local oscillating circuit 35 is connected to the mixing circuit 34. A television signal inputted in an input terminal 36 through a cable not shown is mixed by the mixing circuit 34 with an oscillation signal generated by the local oscillating circuit 35 to provide an intermediate frequency, which is outputted from an output terminal 37 as an intermediate frequency signal.
The input tuning circuit 31 has a varactor diode not shown and selects the frequency signal of a desired channel by a tuning control voltage to be supplied to this varactor diode. In addition, in the frequency of a channel to be received, the input tuning circuit 31 matches the impedance of the cable not shown with the input impedance of the high frequency amplifying circuit 32 and inputs the result into the next high frequency amplifying circuit 32.
The high frequency amplifying circuit 32 is a variable gain amplifying circuit and has a control terminal 38 to which an automatic gain control (AGC) voltage from a wave detector not shown is applied. The interstage tuning circuit 33 is constituted by a double tuner in order to largely attenuate signals of channels adjacent to the desired channel at up and down sides thereof.
Because the intermediate frequency to be outputted from the mixing circuit 34 is of a band of several tens MHz (50 MHz in the Japanese standard and 40 MHz in the US standard), the oscillation frequency generated by the local oscillating circuit 35 is away from the frequency of the channel to be received by a frequency equivalent to the intermediate frequency.
In a CATV system, subscribers thereto watch the television for receiving CATV by individually connecting the television to the CATV cable.
If the tuner having the above-mentioned constitution is used for CATV reception, problems that follow are presented. First, television receivers of many subscribers are connected to the CATV cable as a load. Therefore, the input impedance of one television receiver is a part of the signal source impedance for another television receiver. At this moment, the input impedance of another channel receiving another channel, namely the signal source impedance viewed from a certain receiver receiving a particular channel is different from the input impedance of that receiver. Thus, the conventional tuners affect the signal source impedance of each other and provide a mismatch between the signal source impedance and the receiver input impedance, thereby preventing normal reception.
The oscillation signal to be inputted from the local oscillating circuit 35 to the mixing circuit 34 provides a frequency conversion function in the mixing circuit 34 and passes from the interstage tuning circuit 33 to the high frequency amplifying circuit 32 to the input tuning circuit 31. Then, this oscillation signal, because the frequency thereof is close to the frequency of a television signal, cannot be sufficiently attenuated by the interstage tuning circuit 33 and the input tuning circuit, thereby leaking to the CATV cable not shown through the input terminal 36. The leaking oscillation signal is inputted in the tuners of other subscribers, thereby interfering the reception by these tuners.
Further, the high frequency amplifying circuit is arranged in the foremost stage of the tuner, so that the signal to be inputted in the input terminal of the high frequency amplifying circuit is not controlled in intensity and therefore is wide in range and excess in magnitude. This can cause a distortion at the input terminal of the high frequency amplifying circuit, thereby degrading reception quality.